The long range purpose of this project is to investigate the role of type C retroviruses as an etiologic agent and a vector of genetic information for neoplasms and the use of viral mutants to analyze the mechanism of regulation of gene expression associated with cell differentiation and oncogenesis. The topics of current interest are: 1) molecular cloning and characterization of viral DNA derived from amphotropic virus strains isolated in this laboratory, 2) characterization and nuclotide sequencing of polyoma virus mutants that can replicate in embryonal carcinoma and trophoblast cells, 3) role of methylation affecting the expression of type C retroviruses and oncogenes in embryonal carcinoma and trophoblast cells, 4) in vitro transmission studies of the human T cell leukemia virus, and 5) studies on oncogenes of human choriocarcinoma, and other tumors.